Define an expectation with name desc. Name gets morphed to a
proper test method name. For some freakish reason, people who write specs
don’t like class inheritence, so this goes way out of its way to make sure
that expectations aren’t inherited.

This is also aliased to specify and doesn’t require a desc
arg.

Hint: If you do want inheritence, use minitest/unit. You can mix
and match between assertions and expectations as much as you want.

Register a new type of spec that matches the spec’s description. This
method can take either a Regexp and a spec class or a spec class and a
block that takes the description and returns true if it matches.